Aerospace sealants must satisfy demanding mechanical, chemical, and environmental requirements. The sealants can be applied to a variety of surfaces including metal surfaces, primer coatings, intermediate coatings, finished coatings, and aged coatings. Sealants comprising sulfur-containing prepolymers that exhibit acceptable fuel resistance, thermal resistance, and flexibility for aerospace applications are described, for example, in U.S. Pat. No. 6,172,179. In sealants such as those described in U.S. Application Publication Nos. 2006/0270796, 2007/0287810, and 2009/0326167, a sulfur-containing polymer such as a thiol-terminated polythioether prepolymer can be reacted with a polyepoxide curing agent in the presence of an amine catalyst to provide a cured product. These systems are useful as sealants and can meet the demanding performance requirements of the aerospace industry including fuel resistance. Cured aerospace sealants must exhibit acceptable tensile strength, elongation, and adhesion to a variety of aerospace substrates and must maintain these properties following exposure to aviation fluids.
Reducing the weight of aerospace components including coatings and sealants can significantly increase fuel economy. To reduce the weight of aerospace vehicles low density filler can be added to a coating composition. Coatings and sealants having a specific gravity of about 1 are commercially available. To further reduce the weight of aerospace vehicles it is desirable that coatings and sealants have a specific gravity less than 1.
U.S. Pat. Nos. 8,816,023 and 8,993,691, each of which is incorporated by reference in its entirety, disclose low density aerospace sealant compositions characterized by a specific gravity of about 1. These compositions include low density filler formed from thermally expandable thermoplastic microcapsules that have an exterior coating of an aminoplast resin functionalized with a polythiol. Cured compositions comprising the thiol-functionalized coated microcapsules exhibited superior solvent resistance as determined by percentage swell measurements following immersion in methyl ethyl ketone or in Jet Reference Fuel (JRF) Type I for 7 days at 140° F. (60° C.) compared to lightweight sealants made using thermally expandable thermoplastic microcapsules without the polythiol-functionalized aminoplast resin coating. Other important properties such as tensile strength, elongation, and adhesion following immersion in JRF Type I and/or NaCl were not evaluated. The sealant compositions disclosed in U.S. Pat. Nos. 8,816,023 and 8,993,691 were limited to compositions having a specific gravity of about 1 and a loading of low density microcapsules of about 30 vol %, where vol % is based on the total volume of the sealant composition or about 2 wt % where wt % is based on the total weight of the composition.
Increasing the loading of light weight fillers can reduce the properties of a cured sealant. For example, the cohesion strength of a sealant is primarily imparted by the resins, and as the vol % of a filler increases, for example, to 50 vol % of the composition, there is less resin available to support the physical properties of the cured sealant. Also, with increasing filler content, the interface between the filler particles and the binder increases and can provide additional failure sites. The binder and additives must impart sufficient integrity for the sealant to adhere to both the surface of the substrate and to the incorporated filler particles.
Therefore, it is desirable to provide low density sealants having a specific gravity less than 0.9 that meet aerospace sealant performance requirements.